Department of Obstetrics and Gynecology and Reproductive Biology, 75 Francis Street, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115, USA.
Cell Div. 2011 Aug 25;6:17. doi: 10.1186/1747-1028-6-17.
An oocyte undergoes two rounds of asymmetric division to generate a haploid gamete and two small polar bodies designed for apoptosis. Chromosomes play important roles in specifying the asymmetric meiotic divisions in the oocytes but the underlying mechanism is poorly understood.
Chromosomes independently induce spindle formation and cortical actomyosin assembly into special cap and ring structures in the cortex of the oocyte. The spindle and the cortical cap/ring interact to generate mechanical forces, leading to polar body extrusion. Two distinct force-driven membrane changes were observed during 2nd polar body extrusion: a protrusion of the cortical cap and a membrane invagination induced by an anaphase spindle midzone. The cortical cap protrusion and invagination help rotate the spindle perpendicularly so that the spindle midzone can induce bilateral furrows at the shoulder of the protruding cap, leading to an abscission of the polar body. It is interesting to note that while the mitotic spindle midzone induces bilateral furrowing, leading to efficient symmetric division in the zygote, the meiotic spindle midzone induced cytokinetic furrowing only locally.
Distinct forces driving cortical cap protrusion and membrane invagination are involved in spindle rotation and polar body extrusion during meiosis II in mouse oocytes.
卵母细胞经历两轮不对称分裂，以产生一个单倍体配子和两个设计用于凋亡的小极体。染色体在指定卵母细胞中的不对称减数分裂中起重要作用，但相关机制尚不清楚。
染色体独立地诱导纺锤体的形成和皮质肌动球蛋白组装成卵母细胞皮质中的特殊帽和环结构。纺锤体和皮质帽/环相互作用产生机械力，导致极体挤出。在第二次极体挤出过程中观察到两种不同的力驱动的膜变化：皮质帽的突起和由后期纺锤体中部诱导的膜内陷。皮质帽的突起和内陷有助于使纺锤体垂直旋转，从而使纺锤体中部能够在突起帽的肩部诱导双侧沟，导致极体的分离。有趣的是，虽然有丝分裂纺锤体中部诱导双侧沟，导致合子中的有效对称分裂，但减数分裂纺锤体中部仅局部诱导胞质分裂沟。
在小鼠卵母细胞的第二次减数分裂中，驱动皮质帽突起和膜内陷的不同力参与纺锤体旋转和极体挤出。
